1. Field of the Invention
The present invention relates to ring laser gyroscopes including multioscillator ring laser gyroscopes, and more particularly, to a split-gain multioscillator ring laser gyroscope including a system for removal of external scatter by restricting the entry of externally scattered light back into the ring resonator cavity of the ring laser gyroscope.
2. Description of Related Art
Over the past twenty five years the gaseous medium planar ring laser gyroscope has been developed and evolved as a reliable and relatively environmentally insensitive inertial rotation sensor. Planar ring laser gyroscopes of both triangular and square geometries have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning-wheel mechanical gyroscope is its ability to be configured into a truly strapdown system. This provides a system that not only has a much larger dynamic range than the mechanical equivalent but also one free of mechanical bearings, greatly enhancing its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features the expected mean time between failures of most ring laser gyroscope inertial navigation systems is several times longer than the mechanical gyroscopes they replace.
At low rotation rates, the retroscatter from the mirrors couples energy from one of the oscillating beams into the oppositely propagating beam which locks the oscillating frequencies together yielding zero rotation information at low rotation rates. Current operational ring laser gyroscopes having a planar configuration use mechanical dithering schemes to bias the rate sensor to avoid this will known lock-in phenomenon. Mechanical dither is very effective in reducing the effects of lock-in and makes the ring laser gyroscope a successful navigation-grade gyroscope. However the mechanical dither in such a ring laser gyroscope adds a quasi random noise component to the output and can seriously limit the performance unless additional costly techniques are implemented to minimize the effects of the dither. In addition there are some applications where the presence of either body or mirror dither cannot be tolerated such as with certain military requirements, with space borne applications and with systems needing fast updates of position.
With these limitations in mind alternative biasing techniques have been developed using a nonreciprocal Faraday effect created by the application of an axial magnetic field either directly to the gain medium or to a solid glass element placed within the ring laser cavity. The resulting differential phase shift produced between the counterpropagating beams separates their lasing frequencies and shifts the lock-in band to inputs rates higher than those of concern. An example of this technique applied in a so-called multioscillator configuration may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued on Apr. 4th 1989 to Raytheon Corporation (Terry A. Dorschner, inventor). Modern multioscillator gyroscopes use a nonplanar light path to both force the lasing polarizations to be circularly polarized and to create a frequency difference, known as the reciprocal splitting, between the two types of circularly polarized light. In this manner the beam pairs of each polarization can operate without coupling with one another to create two laser gyroscopes operating in the same cavity. When a Faraday element is added, with a properly applied magnetic field, to the cavity the frequencies of the counterpropagating beam pairs for each gyro are split by an equal but opposite amount. Thus the difference of the output beats of the two gyros is substantially independent of the size of the nonreciprocal Faraday bias but is doubly sensitive to rotation. In such a device there are at least four lasing modes: a left-circularly polarized anticlockwise frequency (La), a left-circularly polarized clockwise frequency (Lc), a right-circularly polarized clockwise beam (Rc), and a right-circularly polarized anticlockwise beam (Ra). The nonreciprocal Faraday splitting between the clockwise and anticlockwise beams is typically of the order of 1 MHz. At least four mirrors form the ring resonator path, which encloses a cavity also containing an excited medium in order to provide the necessary gain for the four modes to lase as shown in FIG. 1A. One of the mirrors is semitransparent to allow light to leave the resonator and fall upon a photo detector for signal processing. Demodulator circuitry removes the Faraday bias carrier frequency and leave a beat signal doubly sensitive to rotation compared with the equivalently sized planar dithered RLG.
FIG 1B shows the arrangement of the gain curves 20a and 20b for an alternative form of a multioscillator-type ring laser gyroscope. The operation of this gyroscope is more fully described in the patent application entitled "Split-Gain Multimode Ring Laser Gyroscope and Method" Ser. No. 07/115,018, filed on Oct. 28th, 1987 (Graham Martin, inventor), and assigned to the same assignee of this application. Ser. No. 07/115,018 is currently under United States Patent Office Secrecy Order (Type One Order). A brief explanation of the split-gain gyroscope may be understood by reference to the gain curves 20a and 20b of FIG. 1B. The split-gain gyroscope operates across two longitudinal mode groups encompassing eight possible lasing modes. A uniform axial magnetic field is applied to the entire gain region in the cavity light path resulting in a splitting of the overall gain curve into two parts; one part 20a will provide gain for modes of one helicity only (La and Rc) while the other part 20b will provide gain for the other helicity only (Lc and Ra). If the magnetic field is tuned so that the splitting of the gain curve is substantially equal to the cavity free spectral range then by suitable minor adjustment of the cavity length the gain curves 20a and 20b can be positioned (as shown in FIG. 1B) so that only the La and Rc modes from the first longitudinal mode group laser while only the Lc and Ra modes from the next longitudinal modes group lase. The resulting lasing modes are the same as those found in the multioscillator configuration which uses a intracavity Faraday element but the split-gain light path has no intracavity elements and the equivalent Faraday nonreciprocal splitting has been increased from the previous typical value of around 1 MHz to a value equal to the cavity free spectral range. The latter value depends on the cavity length but is typically around 2 GHz.
All the above multioscillator ring laser gyroscope configurations are non-dithered alternatives to the dithered planar ring laser gyroscope or the mechanical gyroscope. The Split-Gain multioscillator has the distinct advantage over the Faraday biased multioscillator in that no intra-cavity element (a source of scatter and loss error) is required and the large splitting between the lasing modes ensures that conventional backscatter coupling is not present.
However the large mode separation of the split-gain configuration can create error sources in that the two component gyro beam pairs (the La and Rc modes may be one pair and the Lc and Ra are the other) are far enough away in frequency to give imperfect common mode rejection of some effects. Of concern for this disclosure are the effects caused by light, which has exited the cavity through an output mirror, being scattered back into the cavity in the same propagation direction it previously possessed but with a phase shift dependent on its path length outside the cavity. The main source for this type of retroreflection is scattering centers occurring in the path of the exiting light beam after the beam combining optic. The beamsplitter surface in the combining optic allows any backscattered light in the combined beam to be redirected back into the cavity in the forward direction. Such external scattering effects are present in all ring laser gyros whether they are of the planar dithered variety or of the multioscillator type. However in most cases the effects occur equally for the counterprogagating beams and are common-moded out in the gyro beat. This is very true for the two-mode dithered gyro where the counterpropagating beam frequencies are only separated by the effects of rotation and are generally always within 1 MHz of one another. In the Faraday multioscillator the counterpropagating beam frequencies are sometimes separated by up to several MHz by the Faraday-induced nonreciprocal splitting but the effects of the external scatter, although larger than in the dithered gyro, are still too small to cause problems for most applications. However the counterpropagating beam frequencies in the split-gain configuration are equivalently separated by the cavity reciprocal splitting which typically may be several hundred MHz. In this case the external scatter effects are evident as sinusoidal variations in the bias as the temperature of the combining optic mounted on the gyro frame is varied. It is the intent of this disclosure to provide a means to prevent the unwanted external scatter effects in the split-gain gyro and allow the operation of the device as an inertial grade rotation sensor.